The present invention relates to a process for preparing hydrogenation catalysts which contain noble metals and having at least one catalytically active component and optionally promoters and/or modifiers on a support of activated carbon.
The hydrogenation of organic compounds is performed on a large-scale in the presence of catalysts which contain noble metals on activated carbon. An important field of application is for example, hydrogenation of haloamines. The activity and selectivity of these catalysts may be influenced by promoters and modifiers. Promoters reinforce the catalytic effect whereas modifiers alter the selectivity by partially poisoning the catalyst. Depending on the reaction which is desired, specific components may act in one case as promoters and in another case as modifiers. According to German patent 21 50 220, U.S. Pat. No. 3,150,185, German OLS 20 42 368 and German patent application P 42 18 866.0-41 which has still not been laid open to public inspection, suitable noble metals are the metals Pt, Pd, Rh, Ru, Os and iridium. S, Pb, Bi, Ag, Cu and Fe may be used as modifiers.
The support used for these catalytically important components is preferably activated carbon in a finely divided form with an average particle diameter of 15 to 30 .mu.m or as molded items with dimensions of 1 to 5 mm. A variety of such molded shapes such as pellets are known.
As is well known, activated carbon is a natural product and is obtained from carbon-containing materials by charring followed by chemical or gas activation. Suitable carbon-containing materials are various woods such as beech, pine or coconut shells (Ullmann's Encyclopedia of Industrial Chemistry, Weinheim 1985, vol. A5, pages 124-140).
Activated carbon is characterized by a particularly high specific surface area of 500-1500 m.sup.2 /g, which may be determined from nitrogen adsorption isotherms and assessment by the Brunauer, Emmett and Teller (BET) method (DIN 66132). The high specific surface area of activated carbon is based on the large pore volume of up to 1.5 ml/g, more than one third of which usually consists of micropores with pore diameters of less than 2 nm.
Commercial activated carbon has an ash content of up to 20%, which consists of inorganic components from the starting material. For catalytic applications, only activated carbons with the smallest possible ash contents may be used. This is usually achieved by washing the activated carbon with strong inorganic acids such as, for example, hydrochloric acid and nitric acid. These procedures are well known in the art.
In this way both the ash content of the activated carbon support is drastically reduced, especially by the removal of compounds which contain alkaline earth and heavy metals, and the surface of the support is freshly functionalised. These two properties, inter alia, have an advantageous effect on the final catalyst with regard to catalyst activity and product selectivity in the chemical reactions concerned. Typical residual ash contents are less than 2 wt. %.
Richard and Gallezot ("Preparation of highly dispersed carbon supported, platinum catalyst" Stud. Surf. Sci. Catal., 31 (Prep. Catal. 4), 71-81; 1987, Elsevier Science Publishers B.V. Amsterdam) investigated the functionalisation of surfaces of carbon supports using different oxidizing agents. They found that the surface could also be functionalised using sodium hypochlorite or hydrogen peroxide as an alternative to concentrated nitric acid (65%). The number of functional groups obtained, however, was much smaller when using hydrogen peroxide, even when applied in 30% strength aqueous solution for a period of 24 hours, than when using concentrated nitric acid.
To prepare hydrogenation catalysts, an initial activated carbon is used whose residual ash content has been reduced to less than 2 wt. % by washing with nitric acid. Suitable noble metals have to be deposited on this activated carbon. For this as is described, for example, in P 42 18 866.0-41, powdered activated carbon is suspended in water. After the addition of aqueous solutions of water-soluble compounds of noble metals and optionally of modifiers to the activated carbon suspension, noble metals and modifiers are precipitated onto the activated carbon in the form of their barely soluble compounds by the addition of a base to the suspension. Then follows reduction of the catalytically important components, using a reducing agent such as hydrazine, sodium formate, sodium borohydride or formaldehyde.
It has been shown that the hydrogenation catalysts prepared in this way have very large variations in hydrogenation activity and selectivity. This can be attributed to variations in the quality of these activated carbon supports which originate from natural sources.
In particular, the low ash contents and the functionalised surfaces cannot always be set by washing with nitric acid under economically viable, large-scale conditions.
There is generally an optimum for the washing conditions for a specific type of activated carbon. Experience has shown that the performance of the resulting catalysts is negatively affected by washing with too high concentrations of nitric acid and using too long washing times.